1. Field of the Invention
Embodiments of the invention relate to a method for compensating for an acceleration vector offset, a recording medium storing a program for executing the method, and an apparatus adapted to perform the method. In particular, embodiments of the invention relate to method for compensating for an acceleration vector offset of an acceleration sensor adapted to measure acceleration of a hard disk drive (HDD), a recording medium storing a program for executing the method, and an apparatus adapted to perform the method.
This application claims priority to Korean Patent Application No. 10-2005-0107014, filed on Nov. 9, 2005, the subject matter of which is hereby incorporated by reference in its entirety.
2. Description of Related Art
A hard disk drive (HDD) is a recording device adapted to store information. Information is recorded on concentric tracks on each of an upper and a lower surface of at least one magnetic disk. Each disk is mounted on a rotating spindle motor, and the information is accessed by a read/write head mounted on an actuator arm rotated by a voice coil motor (VCM). In order to move the read/write head, the VCM is rotated by applying electrical current. The read/write head reads the information recorded on a surface of a disk by sensing magnetic variations on the surface of the disk. A current is provided to the read/write head in order to record information on data tracks of the disk. The provided current generates a magnetic field for magnetizing the surface of the disk.
HDDs have been continuously reduced in size to the point where they can now be used in portable mobile devices such as laptop computers, MP3 players, cellular phones, and personal digital assistants (PDAs).
Portable mobile devices are frequently carried and thus are at risk of being dropped. Dropping a portable mobile device can cause damage (i.e., shock damage) to heads and disks of an HDD in the portable mobile device. Thus, in a portable mobile device comprising an HDD, the HDD needs to be protected when dropping or another motion that may cause damage to the HDD is predicted.
To protect both an HDD disposed in a portable mobile device and its data, technology that detects when an HDD is hit, dropped, or vibrated, and that unloads the heads of the HDD, if necessary, has been introduced. The purpose of that technology is to protect an HDD from damage that may be caused by a hit, drop, or vibration. An example of such technology is disclosed in published Japanese Patent Nos. 2000-99182 and 2002-8336, the subject matter of which is hereby incorporated by reference. The technology relates to detecting a free-fall state using a free-fall sensor (FFS), and retracting the head(s) of an HDD when the free-fall state is detected.
FIG. 1 is a perspective view of an acceleration detector. The acceleration detector is a 3-axis acceleration detector. Referring to FIG. 1, a FFS 50 comprises a mass 52 and piezo elements 54 attached to mass 52. Mass 52 is subject to movement in the x, y, and z directions in correspondence with motion of an HDD incorporating FFS 50. Movement of mass 52 defines the respective amplitudes of electrical-signals generated by piezo elements 54. A vector of movement and/or a vector of acceleration for mass 52 may be calculated in relation to the respective electrical signals indicating movement in the x, y, and z directions. A free-fall state may be indicated by the calculated vector(s) of movement and acceleration.
FIGS. 2 and 3 are diagrams for explaining a method for detecting a free-fall state for an HDD. A vector of acceleration representing the aggregated values in each of the principal axes of measurement may be used to detect a free-fall state. That is, as conceptually illustrated in FIG. 2, a falling HDD will experience acceleration under the influence of gravity. Detection of this acceleration indicates a a free-fall state for the HDD.
When an acceleration vector is used as a free-fall indicator, it may be calculated as the sum of acceleration vectors in the three principal axes. This aggregate vector value may then be compared to a threshold value “Th” defined in relation to a sample time period “Tfall”. As further illustrated in FIG. 3, when a free-fall state for an HDD is recognized, a free-fall detection signal “DETECT FREE-FALL” is generated. When the free-fall detection signal “DETECT FREE-FALL” is generated, the HDD performs a retract operation adapted to park or unload the read/write head(s) of the HDD.
However, the conventional FFS 50 typically suffers from an acceleration vector offset, which is an amount by which a measured acceleration vector differs from a corresponding actual acceleration vector. The acceleration vector offset is commonly referred to as an “0G offset”, and may be though of as an acceleration vector initially influencing FFS 50 even when this device is at rest. As used herein, the indication “G” denotes the acceleration of gravity (i.e., 9.8 m/sec2). Because the acceleration vector offset is a measurement error related to an actual acceleration measurement, the acceleration vector offset must be compensated for to order to properly detect and indicate a free-fall state using an acceleration vector. The acceleration vector offset may be caused by variations in ambient operating temperature, supply voltage, and manufacturing process.
Each of the three axes of measurement for FFS 50 may include an acceleration value offset. An acceleration value offset is the difference between a measured acceleration value and a corresponding actual acceleration value. An acceleration vector offset, which is the vector sum of the acceleration value offsets for the three axes, may result in false positives or false negatives when detecting whether the FFS 50 is free-falling (i.e., in a free-fall state).
FIGS. 4A and 4B are graphs illustrating false negative and false positive detection of a free-fall state, respectively. False negative detection of free-fall occurs when, as illustrated in FIG. 4A, measured acceleration vectors are offset from corresponding actual acceleration vectors in a positive direction (i.e., each measured acceleration vector is greater than the corresponding actual acceleration vector). False negative detection of free-fall means that, even though the FFS 50 is in the free-fall state, the FFS 50 does not detect that the FFS 50 is in the free-fall state.
False positive detection of free-fall occurs when, as illustrated in FIG. 4B, measured acceleration vectors are offset from corresponding actual acceleration values in a negative direction (i.e., each measured acceleration vector is less than the corresponding actual acceleration vector). False positive detection of free-fall means that the FFS 50 detects that the FFS 50 is in the free-fall state even though it is not.
Thus, the acceleration vector offset suffered by the FFS 50 must be compensated for so that free-fall can be detected accurately so that an HDD will perform retract operations at the appropriate times.